1. Field of the Invention
The present invention relates to power switch systems, and particularly to a power switch system for selectively supplying a primary power and a secondary power.
2. Description of Related Art
In the computer field, USB (Universal Serial Bus) devices, such as USB hard disk, USB digital camera, USB projector etc., are widely used because they are hot-swappable, and provide a fast transmission speed. In these USB devices, a battery power source is usually built therein. When power provided by USB port in the computer is unstable or unavailable, the battery must be switched on to provide power.
Referring to FIG. 3, a typical power switch circuit 100 is shown. The power switch circuit 100 includes a diode D1, a resistor R1, and a transistor Q1. The emitter of the transistor Q1 is connected to a battery power terminal 10. The collector of the transistor Q1 as an output terminal 11 of the circuit 100 outputs the battery power. The anode of the diode D1 is connected to the base of the transistor Q1 and a USB power terminal 12 and is grounded via the resistor R1. The cathode of the diode D1 is connected to the output terminal 11.
When the diode D1 is turned on while the transistor Q1 is turned off, the USB power is output via the output terminal 11. However, if the USB power becomes unstable or unavailable, the transistor Q1 cannot be turned on due to stabilizing by the diode D1. Therefore, the battery power cannot be switched on for providing power, thus a USB device depending on the USB power will not function.
What is needed, therefore, is to provide a power switch system for switching between power sources immediately and efficiently.